This example demonstrates how a water analysis can be (routinely) checked and interpreted by hydrochemistry. We start with a groundwater sample, gw.sol, that belongs to the collection of example files shipped with aqion.

The very first plausibility check inspects the cation-anion balance. The result is displayed as CBE shown in the right image.

In this example the charge-balance error of 0.33% is really small, which indicates an analysis of excellent quality. Nonetheless, there is always the possibility to establish full charge balance, i.e. an equilibrium solution with CBE ≈ 0.

Charge balance can be established only if the corresponding left checkbox is activated. Then you are able to select one parameter from the a dropdown list. Let us select the pH value (default) and click on the next button to start the pH adjustment.

Charge-Balance Adjustment

In this example, full charge balance is established by adjusting the chosen parameter pH from 6.90 to 6.91 (see right screenshot). This is, in fact, a tiny adjustment – due to the good quality of the water analysis data. (In reality we are often faced with analytical data of less accuracy, i.e. CBE ≥ 3%.)

Technically, the program performs two subsequent calculations before the results are displayed in the right panel:

If the water is supersaturated with one or more minerals the corresponding mineral(s) will precipitate (as indicated in Output 2). Precipitation is often accompanied with changes of pH, and these changes are displayed too. (In the present example, however, the amount of precipitated Fe(OH)3 is too small to alter the pH.)

By click on button next you obtain the complete aqueous composition of the input and output waters (Step 3).

Step 3: Output Table

The structure of the output table in the right screenshot is explained here. It contains three water compositions labeled as Input, Output 1 and Output 2:

Input. This column represents the input water (raw data).

Output 1. This column represents the aqueous solution after charge balance adjustment. It is the result of an equilibrium calculation.

Output 2. This column displays the aqueous solution after charge balance adjustment and equilibrium with mineral phases. In this example, Fe almost disappears due to the precipitation of amorphous ferrihydride, Fe(OH)3.

In the second calculation, also redox equilibria are established, which influence the Fe(2)/Fe(3) ratio as well as the disproportionation between nitrate and nitrite.2

Other Data. While the screenshot above displays total concentrations of elements additional information is provided in separate tables (in the upper menu bar):